A typical control circuit for an inductive electrical load, such as an electric motor, will derive power from a single phase mains supply, making use of a diode bridge rectifier and a smoothing capacitor to derive a direct voltage. Capacitively smoothed rectifiers are well known for their high harmonic content, both in relative amplitude and the extent of the harmonics, of the current drawn from the mains supply.
The switched reluctance motor is operable with uni-directional, but pulsating, motor winding currents which are controlled by a power converter which uses semi-conductor switches in known controls. The currents continue to flow, usually only briefly, through diodes in the conventional converter after a switch has ceased to conduct. When a winding current is carried solely by the diodes at least a proportion of the energy in the winding is returned to a smoothing capacitor. The smoothing capacitor serves two main purposes: firstly to accept the energy returned from the motor windings and secondly to provide a relatively smooth direct voltage supply, derived from the single phase alternating current mains supply, for the power converter circuit which provides and controls the pulsating current supplied to the motor windings.
Legislation is being introduced which is designed to set limits on the amplitudes of the currents drawn at other than the fundamental, e.g. mains, frequency. This restricts the power which can be taken from the main supply by a capacitively smoothed diode bridge rectifier arrangement. When the converter power rating is likely to exceed the set limit, additional components are required in the converter circuitry between the mains supply and the smoothing capacitor in order to keep the amplitudes of currents, at frequencies above the fundamental frequency, within any limits laid down.